A capacitive touch device detects a touch point by sensing the capacitive variation of a capacitive touch sensor. For example, referring to FIG. 1, a two-dimension touch sensor 10 has X-direction traces X1-X9 and Y-direction traces Y1-Y8, wherein the traces X1-X9 and Y1-Y9 are not electrically connected to each other. In addition to having their self capacitances, the traces have mutual capacitances therebetween. When contacted by a finger 12, the trace has both of the self capacitance and the mutual capacitance it senses varying. Currently, a sensor structure for the capacitive touch device may use either all point scan or projection scan. The former one is measuring the variations of the mutual capacitances at all trace intersections during a sensing period of a frame, while the latter one is measuring the variations of the self capacitances of all traces during a sensing period of a frame and generating intersecting points by projecting the intersections of the two directions. In sensing the variations of the self capacitances, taking a trace Y7 as an example, an excitation signal Tx is applied to the trace Y7, and a sensed signal Rx fed back by the same trace Y7 is received. The sensed value generated from the sensed signal Rx represents the self capacitance of the trace Y7. When a finger 12 contacts the trace Y7, the ground capacitance of the finger 12 is connected to the self capacitance of the trace Y7 in parallel, so the value now sensed at trace Y7 is different from that obtained when the trace Y7 is untouched. This is therefore a basis for determining whether the trace Y7 is touched. In sensing the mutual capacitance, taking traces X8 and Y7 for example, an excitation signal Tx is applied to the trace Y7 (or X8), and a sensed signal Rx fed back by the corresponding trace X8 (or Y7) is received. The sensed value generated from the sensed signal Rx represents the mutual capacitance between the traces X8 and Y7. When the finger 12 contacts the intersection between the traces X8 and Y7, the ground capacitance of the finger 12 reduces the sensed mutual capacitance, which is a proof of that the intersection of the traces X8 and Y7 are touched.
In sensing process of a capacitive touch sensor, noise interference can bring errors to the sensed value, causing an untouched point to be mistaken as a touched point, or causing the reported coordinates inaccurate, which in turn leads to misoperation. For example, referring to FIG. 1, when a finger 12 touches at a point between the traces X7, X8 and Y7, Y8, the mutual capacitance between the traces X7, X8 and Y7, Y8 or the self capacitance of the trace X7, X8 and Y7, Y8 can be used to calculate the coordinates representing where the finger 12 is. However, if the sensed value is error due to noise interference, the calculated coordinates are deviated from the actual position of the finger 12. If the sensed value of the mutual capacitance between the trace X3 and Y7 or the sensed values of the self capacitances of the traces X3 and Y7 varies over the threshold set by the sensing circuit due to noise interference, the sensing circuit will identify the intersection 14 of the traces X3 and Y7 as another touched point. For eliminating mistakes caused by noise interference, an approach is to use a median filter for filtering signals, yet this can reduce the sensitivity. Another approach is to raise a threshold of the algorithm, but this can make the capacitive touch sensor less supportive to hardware. Yet another approach is to sense the self capacitance or mutual capacitance for several successive times in a frame, and then average the sensed values. For example, referring to the system block diagram of FIG. 2, a capacitive touch panel 16 has a capacitive touch sensor (not shown) connected to a sensing apparatus 18. When a finger or other electrically conductive object touches the capacitive touch panel 16, the capacitance of the capacitive touch sensor changes and this change is sensed by the sensing apparatus 18 and reflected in an output signal So. In the sensing apparatus 18, a control unit 20 controls an excitation unit 22 to apply an excitation signal Tx to the capacitive touch sensor, a sampling unit 24 samples the sensed signal Rx fed back by the capacitive touch sensor, an analog-to-digital converter (ADC) 26 converts the sampled signal Rx into a sensed value Sd, and an averaging unit 28 averages sensed values Sd obtained in a given time period to generate an output signal So. In sensing the same self capacitance or mutual capacitance, referring to FIG. 3, according to one scan frequency, the excitation signal Tx is applied successively for several times and the sensed signal Rx is sampled successively for several times, and one sensed value Sd is obtained in each sensing cycle Ts, while the average of such sensed values Sd is the output signal So. The more times of repeatedly sensing the same self capacitance or mutual capacitance, the smaller the interference caused by noise of a certain frequency to the output signal is, yet the lower the frame rate is, making the response of the capacitive touch device slower and bringing about users' unsmooth operation.
Conventional solutions all compromise other parameters, and therefore it is desired a sensing method and apparatus for suppressing noise interference without compromising other parameters.